B Lymphocyte Stimulator (BLyS™) protein is a member of the tumor necrosis factor (“TNF”) superfamily that induces both in vivo and in vitro B cell proliferation and differentiation (Moore et al., Science 285: 260-263 (1999)). B Lymphocyte Stimulator is distinguishable from other B cell growth and differentiation factors such as IL-2, IL-4, IL-5, IL-6, IL-7, IL-13, IL-15, CD40L, or CD27L (CD70) by its monocyte-specific gene and protein expression pattern and its specific receptor distribution and biological activity on B lymphocytes. B Lymphocyte Stimulator expression is not detected on natural killer (“NK”) cells, T cells or B cells, but is restricted to cells of myeloid origin. B Lymphocyte Stimulator expression on resting monocytes is upregulated by interferon-gamma (IFN-gamma). The gene encoding B Lymphocyte Stimulator has been mapped to chromosome 13q34.
B Lymphocyte Stimulator is expressed as a 285 amino acid type II membrane-bound polypeptide and a soluble 152 amino acid polypeptide (Moore et al., 1999 supra). The membrane-bound form of B Lymphocyte Stimulator has a predicted transmembrane spanning domain between amino acid residues 47 and 73. The NH2-terminus of the soluble form of B Lymphocyte Stimulator begins at Ala134 of the membrane-bound form of B Lymphocyte Stimulator. Soluble recombinant B Lymphocyte Stimulator has been shown to induce in vitro proliferation of murine splenic B cells and to bind to a cell-surface receptor on these cells (Moore et al., 1999 supra). Soluble B Lymphocyte Stimulator administration to mice has been shown to result in an increase in the proportion of CD45Rdull, Ly6Dbright, (also known as ThB) B cells and an increase in serum IgM and IgA levels (Moore et al., 1999 supra). Thus, B Lymphocyte Stimulator displays a B cell tropism in both its receptor distribution and biological activity.
Based upon its expression pattern and biological activity, B Lymphocyte Stimulator has been suggested to be involved in the exchange of signals between B cells and monocytes or their differentiated progeny. The restricted expression patterns of B Lymphocyte Stimulator receptor and ligand suggest that B Lymphocyte Stimulator may function as a regulator of T cell-independent responses in a manner analogous to that of CD40 and CD40L in T cell-dependent antigen activation. As such, antibodies and related molecules that immunospecifically bind to B Lymphocyte Stimulator may find medical utility in, for example, the treatment of B cell disorders associated with autoimmunity, neoplasia, or immunodeficiency syndromes.